1. Field of the Invention
The present invention relates to an intermediate telephoto lens system with an angle-of-view of about 24xc2x0 to 27xc2x0 (corresponding to a focal length of about 90 mm to 100 mm in a photographing lens system for a 35 mm-format camera), which is suitable for a single lens reflex camera, a video camera and the like.
2. Description of the Related Art
An intermediate telephoto lens system with an angle-of-view of about 240 to 270 has been proposed in, for example, U.S. Pat. No. 4,508,434 Japanese Unexamined Patent Publication (J.U.P.P.) No. Sho-62-244010, and J.U.P.P. No. Hei-5-224119; however, the intermediate telephoto lens systems disclosed therein cannot correct astigmatism and chromatic aberration sufficiently. In particular, the intermediate telephoto lens system disclosed in J.U.P.P. No. Hei-5-224119, a floating mechanism is employed for improving the optical performance at close distances; however, the floating mechanism is complicated, which greatly increases the production costs. Despite employing the floating mechanism, the amount of change in aberrations at closer distances becomes noticeable, in particular, fluctuation of astigmatism becomes larger.
It is an object of the present invention to provide an intermediate telephoto lens system which has at least the following features:
(i) chromatic aberration is made less noticeable;
(ii) astigmatic difference and field curvature are small;
(iii) the amount of change in aberrations occurred at infinity toward a close distance corresponding to a magnification of about xe2x88x921/5 is small; and
(iv) an angle-of-view is about 24xc2x0 to 27xc2x0.
In order to achieve the above-mentioned object, there is provided an intermediate telephoto lens system including a positive first lens group 1, and a positive second lens group 2, in this order from the object. The first lens group 1 includes a positive 1-1 lens element (a first lens element) having stronger positive power on the object-side surface thereof, a positive 1-2 lens element (a second lens element) constituted by a positive meniscus lens element having positive power on the object-side surface, a negative 1-3 lens element (a third lens element) constituted by a negative meniscus lens element having the concave surface facing toward the image, an aperture stop (xe2x80x98Stopxe2x80x99 in Tables), cemented lens elements constituted by a negative 1-4 lens element (a fourth lens element) having a concave surface facing toward the object and a positive 1-5 lens element (a fifth lens element), and a positive 1-6 lens element (a sixth lens element), in this order from the object. The second lens group 2 includes a negative 2-1 lens element (a seventh lens element) constituted by a negative meniscus lens element having the convex surface facing toward the object, and a positive 2-2 lens element (an eighth lens element), in this order from the object. Upon focusing, only the first lens group 1 moves along the optical axis. Furthermore, the intermediate telephoto lens system satisfies the following condition:
(xcexdxe2x80x21-1+xcexdxe2x80x21-2)/2 greater than 102xe2x80x83xe2x80x83(1) 
wherein
xcexdxe2x80x21-1 designates the dispersion ratio of the positive 1-1 lens element;
xcexdxe2x80x21-2 designates the dispersion ratio of the positive 1-2 lens element;
(xcexdxe2x80x2=(ndxe2x88x921)/(ngxe2x88x92nF) 
nd designates the refractive index of the d-line with respect to each lens element;
ng designates the refractive index of the g-line with respect to each lens element;
nF designates the refractive index of the F-line with respect to each lens element.
The intermediate telephoto lens system according to the present invention preferably satisfies the following condition:
xe2x88x921.10 less than f/f2-1 less than xe2x88x920.05xe2x80x83xe2x80x83(2) 
wherein
f2-1 designates the focal length of the negative 2-1 lens element.
The intermediate telephoto lens system according to the present invention can satisfy the following condition:
xe2x88x920.3 less than (xcfx864+xcfx865)f less than 0xe2x80x83xe2x80x83(3) 
wherein
xcfx864 designates the power of the image-side surface of the positive 1-2 lens element, which is defined as xcfx864=(1xe2x88x92n1-2)/r4;
n1-2 designates the refractive index of the 1-2 lens element with respect to the d-line, and
r4 designates the radius of curvature of the image-side surface of the positive 1-2 lens element; and
xcfx865 designates the power of the object-side surface of the negative 1-3 lens element, which is defined as xcfx865=(n1-3xe2x88x921)/r5;
n1-3 designates the refractive index of the negative 1-3 lens element with respect to the d-line, and
r5 designates the radius of curvature of the object-side surface of the negative 1-3 lens element.
The intermediate telephoto lens system according to the present invention preferably satisfies the following condition:
0.2 less than f/f1-6 less than 0.4xe2x80x83xe2x80x83(4) 
wherein
f1-6 designates the combined focal length of the positive 1-1 lens element to the negative 1-3 lens element.
The intermediate telephoto lens system according to the present invention can satisfy the following condition:
0.15 less than (r7xe2x88x92r9)/f less than 0.45xe2x80x83xe2x80x83(5) 
wherein
r7 designates the radius of curvature of the object-side surface of the negative 1-4 lens element; and
r9 designates the radius of curvature of the image-side surface of the a positive 1-5 lens element.
Preferably, the intermediate telephoto lens system according to the present invention satisfies the following condition (6):
xe2x88x9220 less than "sgr"2-1 less than xe2x88x921xe2x80x83xe2x80x83(6) 
wherein
"sgr"2-1 designates the shape factor of the negative 2-1 lens element, which is defined as "sgr"2-1=(r13+r12)/(r13xe2x88x92r12);
r12 designates the radius of curvature of the object-side surface of the negative 2-1 lens element, and
r13 designates the radius of curvature of the image-side surface of the negative 2-1 lens element.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-252828 (filed on Aug. 23, 2000) which is expressly incorporated herein by reference in its entirety.